Feedback image enhancement process

ABSTRACT

A reiterative feedback image enhancement process for enhancing the details of photographic images. In accordance with a preferred embodiment of the invention, the feedback image enhancement process comprises three successive cycles of three successive steps (a), (b), and (c), followed by a final operation. In step (a) of the first cycle, a positive image is derived from an original negative image of a scene, in step (b) a blurred negative image is produced from the positive image, and in step (c) a mask image is produced from the blurred negative image. In each of the second and third cycles, a positive image is produced in step (a) from the original negative image and the mask image produced in the previous cycle, a blurred negative image is produced in step (b), and a mask image is produced in step (c). In the final operation, an enhanced positive image is produced from the original negative image and the mask image produced in the third cycle.

United States Patent [72] inventors George A. Biernson [45] PatentedOct. 26, 1971 [73] Assignee Sylvania Electric Products Inc.

[54] FEEDBACK IMAGE ENHANCEMENT PROCESS 16 Claims, 2 Drawing Figs.

Primary Examiner-George F. Lesmes Assistant Examiner-John R. MillerAttorneys-Norman .l. O'Malley, Elmer J. Nealon and Peter XiarhosABSTRACT: A reiterative feedback image enhancement process for enhancingthe details of photographic images. In accordance with a preferredembodiment of the invention, the feedback image enhancement processcomprises three successive cycles of three successive steps (a), (b),and (c), followed by a final operation. in step (a) of the first cycle,a positive image is derived from an original negative image ofa scene,in step (b) a blurred negative image is produced from the posi- [52] US.Cl 96/27, i imagg d i t (c) a mask image is produced from the 96/44355/80 blurred negative image. In each of the second and third cycles,[51] It. a o itive image i produced in step from the original nega. [50]Field of Search 96/27, 44 five image and the mask image produced in theprevious cyde a blurred negative image is produced in step (b), and amask [56] References Cited image is produced in step (c). In the finaloperation, an UNITED STATES PATENTS enhanced positive image is producedfrom the original nega- 3,085,469 4/1963 Carlson 260/345.l X tive imageand the mask image produced in the third cycle.

CYCLE I CYCLE 2 CYCLE 3 STEP(o) i i l i I I I N I "11111111: I NEGATIVEIN) I IIIIII'IIII I MASK 'M (2) IIIIJJIIIII I MASK STEPM (SPATIAL P( I)referenced... POSITIVE rIEeArIvE L J STEP (c) \FLT. ITMTIITTTQF '1 m2)1m POSITIVE I II l l P(2)}| m sEmIEIIcE H2) I l I ena G I I e I IN FlERING 8(2) I BLURRED BISi I swam-:0

iiiiBLURREDI 9(2) IIIIIIII'IA l NEGATIVE 5(3) n I I-: I NEGATIVE I #2:!NEGA IvE I L J Haw :1: Posmve I I I \FILLUMINATlON i I {2 POSITIVE$\ PtII} l 3 POSITIVES IN SEQUENCE l NEGATIVE NEGATIVE FILLUMINATION 'lBLURRED l BLURRED (CONTACT B (I) l l NEGATIVE I NEGATIVE PRINT) M(|) I2: I ms m2) E22: MASK I Mt!) 4:1: I MASK I k l t /I I L. L ...1 IL ilLLUMINATION I LEGEND BEA]: M (3) ro'e'oto'e'c'o'o'u MASK STEP N:IIEHZZZHJ I NEGATIVE UNEXPOSED FILM EP l::" I ENHANCED EXPOSED NEGATIVEL .l Posmve EXPOSED POSITIVE PAIENTEDncT s 1911 SHEET 2 BF 2 DIFFUSERADIATOR SPATIAL FILTER MASK LENS 23M INVENTORS GEORGE A.BIERNSON BYRAYMOND EULING PAUL W. JONES fie AGENT FEEDBACK IMAGE ENHANCEMENTPROCESS BACKGROUND OF THE INVENTION The present invention relates toimagery processing and, moreparticularly, to a feedback imageenhancement process for enhancing the details of photographic images.

High-quality photographic film is available that can record clearlyphotographic negative images over a range of light intensities of about1,000: 1. Such a high range of light intensities is needed forphotographing high-contrast outdoor scenes in order to record detaileffectively both in brightly lighted areas and dimly lighted areas. Foreffective viewing, it is usually desirable to derive positive paperprints from negatives of such scenes. However, the range of lightintensities that can be recorded clearly on photographic printing paperis quite low, often not much greater than 1021, and so a great deal ofimagery detail in a negative of a high-contrast scene can be lost inmaking a positive print therefrom. To avoid this loss of imagery detail,the range of the imagery data recorded on the photographic negative isoften compressed so that it can be conveyed effectively on aphotographic medium of low dynamic range. This compression of the rangeof the imagery data is referred to herein as image enhancement."

The basic philosophy underlying image enhancement is that byilluminating the dark areas of a negative strongly, and the light areasweakly, the operating range of illumination applied to expose a positivecan be made to be much lower than the range of transmissivity values ofthe negative. Ideally, the illumination is varied gradually over thenegative in order that the pattern of the illumination does not itselfcontribute a artificial detail to the image recorded on the positive.Thus, the positive records an enhanced image of relatively low operatingrange. The same image enhancement effect can also be achieved by placinga medium of nonuniform transmissivity between the negative and a filmand uniformly illuminating the negative to expose the film through themedium of nonuniform transmissivity. The medium of nonuniformtransmissivity operates to attenuate weakly the light from the darkareas of the negative and to attenuate weakly the light from the darkareas of the negative and to attenuate greatly the light from the lightareas of the negative.

The approximate effect of the above-described image enhancement, whetherusing varying amounts of illumination or a medium of nonuniformtransmissivity, is to compensate to a large extent for variations ofillumination on the original scene. Consequently, the enhanced imagerecorded on the positive is similar to what would be recorded withoutimage enhancement if the illumination on the original scene were muchmore uniform.

Various conventional prior art methods and apparatus have been employedheretofore for enhancing photographic images to compress the range ofthe photographic data contained in a negative or a positive. Twomethods, one photographic in nature and the other electro-optical innature, have been widely used to implement image enhancement. Each ofthese methods can be employed to produce an enhanced negative from anoriginal positive. For simplicity,, the following discussion willconsider only the former alternative; however, the latter alternative isessentially the same.

In the photographic method of image enhancement, a negative of a sceneis uniformly illuminated and the image impressed thereon is blurred" bysuitable photographic apparatus. An unexposed film spaced from thenegative is exposed with the blurred image and then developed to form ablurred positive, the blurred positive being being designated a "mask."The mask is characteristically dark in the regions where the originalnegative is light, and light in the regions where the original negativeis dark. The mask is then superimposed on the original negative, and themask-negative combination is uniformly illuminated. The combined imagesof the mask and the original negative are then focused onto an unexposedfilm spaced from the mask-negative combination, which film is exposedwith the images and then developed to represent the desired enhancedpositive." Since the abovedescribed mask is dark in the regions wherethe original negative is light, and vice versa, the range of lightintensities focused onto the second positive, i.e., the enhancedpositive, can be much lower than the range of transmissivity values ofthe enhanced positive. in other words, the range of imagery data iscompressed. Moreover, since the image on the mask is blurred, it doesnot add very much imagery detail to the enhanced positive.

The blurred" positive in the above-described photographic method isoften formed by using suitable photographic apparatus to expose adefocused" image of the negative on the film used to form the positivemask. The primary effect of defocusing the image on the negative is toattenuate the high spatial frequency components of the image and therebyto blur the sharp lines of demarcation between the light and dark areasof the image. Therefore, the blurring operation represents low-passspatial filtering. There are many other ways that this low-pass spatialfiltering of the image can be implemented which have different effectson how the components of the image are modified. However, thesedifferences do not alter the basic principle of image enhancement.

As a variation of the above-described method for deriving an enhancedpositive, a mask is obtained as in the abovedescribed method (byexposing and developing a film with a blurred negative image), theoriginal negative is spaced from the mask, and the mask illuminated sothat the illumination falling on the negative is a defocused (orblurred) image of the mask. The image of the illuminated negative isthen focused onto film to expose an enhanced positive. The aboveoperation provides two stages of low-pass spatial filtering, the firststage of low-pass spatial filtering operating between the negative andthe mask, and the second stage then operating between the mask and thenegative.

In the electro-optical prior art method of photographic imageenhancement, an original negative ofa scene is scanned with a broad beamof light from a cathode ray tube. The cathode ray tube is controlled soas to have a scan rate which is varied in accordance with the amount oflight energy that passes through the negative. Typically, the beam usedto scan the negative is made much broader than the image detail on thenegative but much narrower than the negative itself. The negative isscanned with the light beam and the light energy passing through thenegative is sampled by a photodetector to control appropriately the scanrate of the cathode-ray tube. More particularly, the scan rate of thecathode-ray tube is controlled such that the greater the amount of lightenergy measured by the photodetector, the faster is the scan rate. Thevariations in light energy therefore cause the beam to scan rapidly overthe low-density areas (light areas) of the negative and slowly over thehigh-density areas (dark areas). The illuminated image produced byscanning the negative with the light beam is focused onto suitableunexposed film, which is exposed by the image and then developed to formthe enhanced positive.

Although the electro-optical method of image enhancement is implementedin quite a different manner from the basic photographic method describedabove, the two methods are mathematically equivalent. More specifically,it can be demonstrated that the same pattern of light energy applied tothe negative by the cathode-ray tube can be achieved by the photographicmethod if two stages of low-pass spatial filtering are employed asdescribed hereinabove, that is, between the negative and the mask andthen between the mask and the negative,

The image enhancement techniques and methods described above are fairlyeffective in compressing the range of imagery data in many photographicsituations. However, they have rather severe weaknesses which restricttheir usefulness in especially critical photographic applications. Inparticular, use of the above techniques and methods results in a loss ofimagery detail in regions of a photographic image close to abruptdiscontinuities of lighting on the original scene. A fundamentaltheoretical weakness of the above-described image enhancement is thatthe image on the negative is low-pass spatially filtered in forming themask. Spatial filtering of the image on the negative is undesirablebecause the range of transmissivity values of the negative is generallyvery large and, therefore, the image cannot be processed in the mosteffective fashion by spatial filtering. More particularly, the problemis that spatial filtering adds together contributions from differentparts of the image. Therefore, in regions of the image near sharpdiscontinuities of lighting, the spatial filtering adds togethercontributions from very light and very dark areas of the negative, withthe result that the contributions from the dark areas are swamped out bythose from the light areas. Consequently, the image enhancement is poorin these regions, and imagery detail is often lost.

Another method which has been used for enhancing" photographic imageswhich merits brief discussion is commonly known as dodging or shading."In this method, a photographer prepares masks of varyingtransmissivities cut to the shape of the light areas of the negative,and places them between the negative and the positive that is beingexposed. The masks are moved (or dodged") during the exposure, so thattheir outlines do not occur as detail on the positive. The effect of themasks is to attenuate the light from the low-density areas of thenegative and thereby compress the range of the exposure values appliedto the positive. A general disadvantage of the dodging method is thatthe masks are prepared in accordance with the qualitative judgment ofthe photographer. Consequently, parts of the imagery detail on thenegative may be lost or obscured, or the imagery detail may be confusedby undesirable artifacts resulting from use of the masks. Therefore, theeffectiveness of the dodging" method has been quite limited in manyphotographic applications where clarity and correctness of detail arestrict requirements.

SUMMARY OF THE INVENTION Briefly, the present invention relates to areiterative feedback image enhancement process which comprises aninitial cycle followed by at least one subsequent cycle and a finaloperation. In the initial cycle of the reiterative feedback imageenhancement process, an original image relating to a scene is producedfrom which a compressed image is derived, the values of the compressedimage being a fixed function of the corresponding values of the originalimage. A filtered image is then formed by low-pass spatially filteringthe compressed image. The initial cycle is completed by deriving a maskimage from the filtered image, the values of the mask image being afixed function of the corresponding values of the filtered image.

In each subsequent cycle of the reiterative feedback image enhancementprocess, a compressed image is derived from the original image and themask image derived in the preceding cycle, the values of the compressedimage being a fixed function of the product of the corresponding valuesof the original image and the mask image derived in preceding cycle. Afiltered image is then formed by low-pass spatially filtering thecompressed images derived in the present cycle and all previous cycles,the values of the filtered image being a weighted average of thecorresponding values of the low-pass spatially filtered compressedimages derived in the present and all previous cycles. The subsequentcycle is completed by deriving a mask image from the filtered imageformed in the preceding step, the values of the mask image being a fixedfunction of the corresponding values of the filtered image formed in thepreceding step.

To conclude the reiterative feedback image enhancement process, thefinal operation is performed which comprised the step of deriving anenhanced image from the original image and the mask image derived in thepreceding cycle, the values of the enhanced image being a fixed functionof the product of the corresponding values of the original image and themask image derived in the preceding cycle.

In the preceding brief description of the reiterative feedback imageenhancement process of the present invention, such terms as image,"fixed function, fixed function of the product," and weighted average"have been employed. In order to more clearly understand and appreciatethe specific nature of the present invention, a brief discussion of theabove terms as to their meanings may be helpful.

The term image," in the context of the present invention, is intended tomean a two-dimensional array of data points having real values. An imagemay be conveyed in many ways, for example, on film as the transmissivityvalues of the exposed film. Alternatively, if the film is illuminated byeven illumination, the intensity values of the light emanating from thefilm may also represent an image the values of which are proportional tothe corresponding values of the image represented by the transmissivityvalues of the film. As still another example, an image may be conveyedon a cathode-ray tube as a two-dimensional array of brightness values.

To define the terms fixed function, fixed function of the product" andweighted average," three images A, B, and C, each having a coordinatepoint (x, y) and a respective value Z Z,,, and Z, will be considered.Thus, in the context of the present invention, if a value of the image Bat the coordinate point (x, y) is a fixed function of a value of theimage at the corresponding coordinate point (x, y), this means that Zf(Z,,), where f(Z,,) is dependent only on the value z and is independentof any other variable including time and the coordinate point (x, y).Similarly, if a value of the image C at the coordinate point (x, y) is afixed function of the product of the values of the images A and B at theassociated coordinate points x, y), this means that Z,=flZ,,-Z,,), where[(2 2 is dependent only on the product (2 -2,) and is independent of anyother variable including the individual 1,, and Z, values, time, or theindividual coordinate points (x, y). If a value of the image C at thecoordinate point (x, y) is a weighted average of the values of theimages A and B at the associated coordinate points (x, y), this meansthat Z =K,Z,,+K Z where K, and K are constants.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates in detail schematicform the various steps employed in an exemplary three cycles of thefeedback image enhancement process of the present invention and FIG. 2is a schematic representation of apparatus employed to perform low-passspatial filtering in accordance with the present invention.

DESCRIPTION OF A PREFERRED METHOD OF THE INVENTION FUNDAMENTAL EQUATIONSThe principal object of the present invention is to produce an imagewhich represents an enhanced version of an original negative image or anoriginal positive image of a scene. Although the principles and conceptsof the present invention apply equally well to the derivation of anenhanced negative image or to an enhanced positive image of a scene, thefollowing discussion will be directed to the more common and usualsituation in which a negative image of a scene, rather than a positiveimage of a scene, is derived, and from which an enhanced positive imageis produced.

In deriving an enhanced positive image, it is desired that the enhancedpositive image satisfy as closely as possible the following equation:

In equation( I T,, represents the relative transmissivity of theenhanced positive at any point, B is the brightness of the correspondingpoint on the scene that was photographed, T, is

the value at the corresponding point of a low-pass spatially filteredmodification of the relative transmissivity T,, and K and B areconstants. The relative transmissivity T, is defined as the ratio of theactual transmissivity of the enhanced positive to the maximumtransmissivity of the unexposed film used to form the enhancedpositive.In equation (1), the parameter K is termed an enhancement constant, andis a positive constant. In a preferred embodiment of the process of theinvention, the enhancement constant K has a value typically about 1.5,the larger the value of the enhancement constant K, the

greater is the compression of the photographic data achieved by thefeedback image enhancement process of the present invention. Theparameter B, in equation (1) is a constant which pertains to therelative transmissivity t, of the enhanced positive."Preferably, thevalues of the constant B is chosen such thatthe average value of therelative transmissivity T, over the enhanced positive is approximatelyequal to 0.5. Theoretical and experimental studies have shown that highquality image enhancement is achieved if equation l is satisfied.

As indicated above, T, represents a low-pass spatially filteredmodification of the relative transmissivity t,. The spatial filteringoperation at a point having coordinate (x, y) can be described by theintegral equation where x and y' are variables in the x,y coordinatesystem over which the integration is performed, r the radial distancebetween the oint (x,y) and a variable point (x'y'), and is given byrV(.t--.x')"'+(yy'), (3) and W(r) is a weighting function for thespatial filtering operation. The weighting function W(r) in equation(2decreases with increasing value of r.

lt may be noted that equation (1) expresses T, as a function of T, andequation (2) expresses T,as a function of T,. These two equationscannotbe combined to obtain an explicit relationship between the scenebrightness B and the relative transmissivity T, of the enhancedpositive. Therefore, to solve these equations, the present inventionuses a reiterative, feedback process which converges to a steady-statesolution in which equations l) and (2 are both closely approximated. Inthis feedback process, successively closer approximations of the idealrelative transmissivity T, are formed in successive cycles of theprocess.

Equation (1) can also be expressed in the alternative forms T,=( A){l+tan h [-2K T,+ 1 2 log, 3/3, 1; 4 and T,=(%){ I+ tan h [-2 2KT,+l.l5 log(8/8,)1} (5) 5.

To apply these relations to photographic films, it is necessary toconsider the general sensitometric properties of film. The density Dy ofa negative is defined by D Log l/T (6) 6 where T is the absolutetransmissivity of the negative. Over a large part of the operating rangeof most films, the logarithm of the transmissivity is approximately alinear function of the logarithm of the exposure. This region is calledthe linear range of the film. Hence, in the linear range, the density Dyof the negative is approximately a linear function of the logarithm ofexposure E and so can be approximately expressed as D y log EwQ (7)where 7 and C are constants. The parameter y,- is called the gamma" ofthe film. The exposure E applied to the negative at any point isproportional to the brightness B at the corresponding point in thescene. Hence, in the linear range, the density Dy of a point on thenegative is related approximately to the brightness B at thecorresponding point in the scene by the expression D 'y log B+C, 8)Where C is a constant. Substituting equation (8) into equation 5 givesT, 1/2 l+ tan h [2KT,,+ 1.15/ D,-+c,]} (9) where C: gives is a constant.The approximation of equation (9) is considered the best approximationof equation (5) that can be achieved in a practical manner. Therefore,this approximation is regarded as being the desirable goal for thefeedback image enhancement process of the present invention.

Thus, the approximately equal sign" in equation (9) can be replaced byan equal sign and so the process of the present invention operates tosatisfy as closely as possible the equation T,,=(l/2) l tan h[2KT,+1.l5/7 D, -+C (10) To implement equation (10), a mask is preparedwhich is then superimposed on the original negative. The density 1), ofthe mask-negative combination is given by D,=D, -+D

Where D is the density of the mask. in order to implement equation (10)by means of the mask, equation (10) is expressed in the forms T,=( 1/2){1+ tan h l .l5/'y (D, +d,)]l (l2) and T,=(l/2) {l tan h [(l.l5/-y (D'+D, +d,,)]}( 13) where d, is a constant. Comparing equations (10) and(13) shows that the density of the mask D may be given by D,.,-- 2/1. 15Kv.t-T,,+d,,=i .74 K ,-T,+d l 4 d M is a constant. As wasmentionedheretofore, the constant B in equation (I) is preferably chosenso that the average value of the relative transmissivity 7, over theenhanced positive is equal to approximately 0.5. Consistent therewith,the constant d, in equation (13) is preferably chosen so that theaverage value of the relative transmissivity T, over the enhancedpositive is equal to approximately 0.5.

Since the image enhancement process of the present invention operates ina feedback manner, it is necessary that damping" be incorporated intothe process in order for the reiteration to converge to a stablesolution. The damping is achieved by providing memory" betweensuccessive cycles of the feedback process. This memory or damping isachieved by a damping computation routine" that combines informationfrom successive cycles, An effective computation for achieving in thedamping is given by u(q)=[ /(2 1(q)+[( )/c2K+l)] u(q The parameter K inequation (15) is the previously mentioned enhancement. constant"; X; andX, are input and output imagery variables, respectively, of the dampingcomputation routine; X,(q) and x,(ql are output variables for the (q)and (q -l) cycles, respectively, of the feedback image enhance mentprocess; and X,- (q) is the input variable for the (q) cycle. Equation(15) expresses the output variable x, for one cycle (q) in terms of theoutput variable for the previous cycle (q-l lf equation (15) is solvedfor'the relationship between input and output variables, the followingequation is derived Equation (16 shows that the output imagery variablex, for the damping computation routine is a weighted average of theinput imagery variables for the latest and all earlier cycles of thefeedback image enhancement process. it can be shown from this equationthat after several cycles, as the process approaches a steady statecondition, X, is approximately equal to X.

The damping computation briefly described above may be convenientlyperformed in terms of the low-pass spatiallyfilter variable T,Specifically, the variable T, may be designated as the input imageryvariable x, and T,as the output imagery variable x, Equations (l3),(l4), and (16) then become, respectively,

cle. In the last cycle of the image enhancement process, the' positiveis taken to be the enhanced positive because it represents the closestapproximation to the ideal enhanced positive image.

DESCRIPTION OF THE FEEDBACK IMAGE ENHANCEMENT PROCESS FIG. 1

Referring to FIG. 1, there is shown in schematic form and in accordancewith a preferred embodiment of the present invention, the various stepsof the photographic feedback image enhancement process. As indicated byFIG. 1, the feedback image enhancement process comprises three cycles ofthree steps (a) through (c), and a final step, which are performed insequence with feedback being provided between the last step (step (c))in each cycle and the first step (step (a)) in the next succeedingcycle, as indicated by the dotted FEEDBACK" arrows. Although threecycles of steps (a) -(c) are shown in FIG. 1, satisfactory photographicfeedback image enhancement can often be achieved by only two cycles. Themaximum number of cycles required to achieve a desired quality of imageenhancement is determined in accordance with the accuracy and precisionrequired in the particular application. Inasmuch as the output of thelast step of each cycle (excepting the last cycle is used in the firststep of the next succeeding cycle, the desired output of the processthat is, the output of the final cycle (Third Cycle), is obtained as aresult of an iterative operation in which the output of each successivecycle represents a closer approximation to the ideal output. Theoperation of the feedback image enhancement process depicted in FIG. 1will now be described.

For simplicity, the photographic steps (a) and (c) of FIG. 1 are assumedto be implemented by contact printing. However, more complicated opticalimaging can be used. In the feedback image enhancement process to bedescribed, different types of films may be used in the three steps (a),(b), and (c), but the same film type is used in the corresponding stepsofthe various cycles (e.g., step (b) of the third cycle uses the samefilm as steps (b) of the first and second cycles.)

First Cycle: In step (a) of the first cycle, a negative transparency Nrecording a photographic image of a scene is uniformly illuminated toexpose a photographic film which is then developed to provide a fistpositive transparency P(l). The photographic film is operated in thenonlinear toe" ofits sensitometric curve, and so the range of imagerydata recorded on the negative N is compressed in this step. Hence, thefirst positive P(l) records a compressed image. Additionally, thetransmissivity values at the points in the first positive P(l) are afixed function of the transmissivity values of the negative N at thecorresponding points. As will be explained more fully hereinafter, thegamma 7,, of the positive P( l) film used in step (a) is preferably,though not necessarily, equal to the reciprocal of the gamma y ofthenegative N. The value of the illumination exposure is adjusted in thisstep so that the relative transmissivity T,, of the first positive P(l)has an average value over the positive preferably equal to T,,=0.5,although this particular value need not be satisfied very accurately. Aswill be shown later, the above conditions cause the process toapproximately satisfy equation (17) for many types of films. (Note thatfor the first cycle, the density of the mask D (q) in equation 17) iszero.)

in step (b), the first positive P(l) is uniformly illuminated and theimage formed thereby is low-pass spatially filtered (i.e., blurred) bysuitable apparatus ofa type such as shown in FIG. 2. Details of thelow-pass spatial filtering apparatus of FIG. 2 will later be presented.The blurred image is projected onto a photographic film which is thendeveloped to provide a first blurred negative B(l). Preferably, thephotographic film in this step is one which has been previously slightlyexposed (or foggedjfi with a uniform illumination. The purpose offogging the film is to cause the film to operate in the desirable partof its sensitometric characteristic. More particularly, the

s 8 fogging and illumination intensities are adjusted in the presentstep so that the film exposed by the illumination is operated in therange of its sensitometric curve where density is approximately a linearfunction of exposure. This condition satisfies equation 18) as will beexplained hereinafter.

In step (c), the first blurred negative EU) is illuminated uniformly toexpose film which is then developed to provide a first mask M( 1 Thetransmissivity values at the points in the first mask M(l) are a fixedfunction of the exposure values of the first blurred negative B(l) atthe corresponding points. The value of the illumination exposure in thisstep is adjusted such that the film operates in the linear range of itssensitometric curve. Therefore, the density d of the mask M( l) is alinear function of the density D of the first blurred negative B( 1which in turn is a linear function ofthe exposure applied to the firstblurred negative B(l). Thus, the density D of the mask is a linearfunction of the exposure applied to the blurred negative B(l), acondition which satisfies equation 18 As indicated by the dottedFEEDBACK" arrow in FIG. 1, the first mask M(l) is then used in step (a)ofthe second cycle of the process. The mask M(l) contains a blurredpositive image, and so is dark in the areas where the negative N islight, and vice versa.

Second Cycle: In step (a) of the second cycle, the first mask M( l) issuperimposed on the negative N, and the combination is uniformlyilluminated to expose a film which is then developed to provide a second(compressed) positive P(2). In this step, the transmissivity values atthe points in the second positive P(2) are a fixed function of theproduct of the transmissivity values at the corresponding points in thenegative N and the first mask M( l As in the first cycle, the value ofthe illumination exposure is adjusted such that the average value overthe image of the relative transmissivity T, of the second positive P(2)is equal to 0.5 To achieve this value of T, =0.5 requires greaterilluminatioii than in the first cycle because of the attenuationprovided by the mask M( l In step (b), the first positive P(l) exposedin the first cycle and the second positive P(2) exposed in the presentcycle are uniformly illuminated in sequence, and the projected imagesare low-pass spatially filtered (by apparatus such as that of FIG. 2 anddirected onto film which has been previously fogged as in the firstcycle. The same amount of fogging is used in this step as in thecorresponding step of the first cycle. The film is then developed toprovide a second blurred negative B(2). it is to be noted that ismultiple spatial filtering apparatus is provided in this step, thelow-pass spatially filtered positive images may be projectedsimultaneously onto the fogged film and in registration with each other,instead of in sequence as described above.

The amount of illumination exposure applied to each of the positivesP(l) and P(2) is determined in accordance with the damping computationroutine of equation 16). For example, if the previously mentioned valueof K=l.5 is assumed, the factor 2K-l )/(2K+1) in equation (16) is equalto one-half. For this condition, the illumination exposure applied to aparticular positive is reduced by a factor of two for each succeedingcycle. in the present case, therefore, the first positive P( 1) receivesone-half the illumination exposure received during the first cycle andthe second positive P(2) receives the same illumination exposure thatthe first positive P(l) was given during the first cycle. Thus, theaverage illumination values at the points in the second blurred negativeB(2) are a weighted average of the illumination values at thecorresponding points in the low-pass spatially filtered positive images.This point is discussed in more detail hereinafter.

In step (c), the second blurred negative B(2) is uniformly illuminatedto expose film which is then developed to provide a second mask M(Z).The transmissivity values at the points in the second mask M(2) are afixed function of the exposure values of the second blurred negativeB(2) at the corresponding points. As in the previous cycle, theillumination exposure is adjusted such that the film used to form thesecond mask M (2) operates in the linear range of its sensitometriccurve.

I The second mask M(2) is then used in step (a) of the third and finalcycle of the process. Third Cycle: in step (a) of the third cycle, thesecond mask (Elli sup qpqtll azi t x lflsvqih combination this step,

in step (b), the positives P(l), P(2),

is uniformly illuminated to expose a film which is then developed toprovide'the third (compressed)-positive P(3). In thetransmissivityvalues at the points in the third positive P(3) are a fixed function ofthe product of the transmissivity values at the corresponding'points inthe negative N and the second mask M(2). As in previous cycles, thevalue of illumination exposure is adjusted such that the average valueof the relative transmissivity T, of the third positive P(3) is 0.5.

and P(3) are uniformly illuminated in sequence, (or' simultaneously),and the projected images are low-pass spatially filtered and directedonto film which has been previously fogged as in the previous cycles.The same amount of fogging is used as in the corresponding steps of theprevious cycles. The film is then developed to provide a third blurrednegative B(3). For the assumed typical I value of K=l.5, theillumination exposure applied to the first positive P(l) is-one-quarterthe illumination exposure applied "thereto in the first cycle; theillumination exposure applied to the second positive P(2) is one-half.that applied to the first positive P(l) in the first cycle; and theillumination exposure applied to the third positive P(3) is equal to theillumination exposure applied to the first positive H1) in thefirstcycle. Thus, the average illumination values at the points in thethird blurred negative 8(3) are a weighted average of the illuminationvalues at the corresponding points in the low-pass spa- -tially filteredpositive images.

In step (c), the third blurred negative 3(3) is uniformly illuminated toexpose film which is then developed to provide a third andfinal maskM(3). The transmissivity values at the points in the third mask M (3)are sure values of the third blurred negative (3) at the corposure isadjusted such that the film used to form the third mask M (3 operates inthe linear range of its sensitometric curve.

Final Step: The process is concluded-by superimposing the third maskM(3) on the starting negative N, and the combina tion is uniformlyilluminated to expose film or photographic printing paper which is thendeveloped to provide an enhanced'positive EP. The enhanced positive EPis a print or transparency of any suitable form or size. In this step,the transmissivity values at the points in the enhanced positive EP area fixed function of the product of the transmissivity values of thenegative N and the third mask M(3) at the corresponding points. I

' In the above-described feedback image enhancement process, care isexercised to keep the images of the various steps appropriatelycontrolled in size and registration. A con-- venient approach is to use1:] contact prints in steps (a) and (c), and to spatially filter in step(b) in such a manner that no change in size results. Appropriateregistration marks can be applied to the films to keep the images inproper registration. Further, it is to be appreciated that enlarging orreduction can be employed at any step of a cycle provided that the sizechange is compensated for by appropriate reduction or enlarging atanother step. m

a fixed function of the exporesponding points. As in previous cycles,the illumination ex- I 10 i h i(l.l5/').v) r+ o)ii The logarithm of theexposure E, applied to the positive is proportional to the density D, ofthe combination. Hence,

r gio( 64E) (21 where C, is a constant that depends on the illuminationexposure applied to the mask-negative combination when exposing thepositive. From equation (2]), equation (20) can be expressed in the formg p=( i Ian 11 /7.\-) gio( s p)li 2) where C, is aconstant. Equation(22) can be expressed alter natively as i1 Ianh ge( 6 P Y. where C is aconstant. Equation (23) can also be expressed as -=l .."1.\-] '1 (24)Equation (24)'relates theexposure E, applied to a positive in step (a)to the relative transmissivity t, of the positive. Commerciallyavailable film often closelyapproximates the following characteristic inthe low-exposure region:

T=[ l.+C E1]l (25') where T is the relative transmissivity of theexposed'film, E is the exposure applied to the film, 'yisthe gamma ofthe film, and C is a constant. Comparing-equations (24) and (25) showsthat the desired characteristic for step (a) can be very adequatelysatisfied by many films where the gamma 7,, of the film used to derivethe positive is given by 7n 77x) Thus, for step (a), a film ispreferably selected for the positive which'has a gamma 7,, equal to thereciprocal of the gamma 7' of the original negative. As statedpreviously, the illumination exposure in step' (a is preferably selectedsuch that the average relative transmissivity T, of the positive overthe whole image is approximately equal to 0.5

Step b g 1 I For simplicity in the ensuing discussion, the relativetransmissivities of the first, second, third, etc. positives (exposed inthe first, second, third, etc. cycles) may be conveniently designated asT,,( l T, (2), T,,(3), etc. respectively. The time integral of theintensity of the illumination pattern emerging from anilluminatedpositive in step (b is proportional to the relative-transmissivity ofthe positive and the illumination exposure applied to the positive. Thetimeintegral of the intensity of the illumination pattern emerging fromthe first positive during the first cycle may be designated as [E,,,, T,(1)]. If losses in the spatial-filtering optics are neglected, the timeintegral of the illumination intensity (which is, by'definition, theexposure).applied to theblui ed negative during step (b) is h( bo 11(where 7,,(1) is the low-pass spatially filtered modification of Thus,from equation (19) u (4) [Ebo (2K +l )/2] '--(q) (Z Near the toe" of thesensitometric curve of most films there is a nonlinear region where thedensity is approximately a linear function of exposure. In some films,this region covers a larger range than in others. In step (b), the filmis preferably operated in this nonlinear region by fogging the film, (toestablish a minimum value of exposure at the lower limit of thisregiom)and by controlling the illumination (so that the maximum exposure doesnot exceed the upper limit of this region). Therefore, the density D ofthe'blurred negative B exposed in step (b is related as follows to theexposure E applied after fogging DB=' ilmwa.

Step The film for producing the mask M is operated completely in itslinear range, and so the density D of the mask M is given by where C isa constant. Substituting equation (31) ir to equation (33) gives D,,,=(1/2 )7 E (2K+1 (SD ISE T,,'+ u (34) where C, is a constant. Equatingequations (18) and (wives 2(1.74)K w BEE lmllrsllm] (35 Equation (35)describes the illumination exposure required for the latest positive instep (b) to achieve a particular value of the enhancement constant K.

A convenient means of setting E for which the relative transmissivity T,is unity, and thereby expose a blurred negative B, and from this producea mask M. A mask is also produced from a blurred negative B which isunexposed except for the fogging. The differences in the densities ofthese two masks, designated as AD is equal to Substituting equation (36)into equation (35) gives AD =3.48 K7,\-/(2K+l (37) Equation (37) allowsthe enhancement constant to be conveniently controlled. For example, ifK =1.5, and 7;- =1, then AD 1.30. Thus, in this case, the exposureillumination for step (b should be adjusted so that the density of themask changes by 1.3 when the relative transmissivity of the positive ischanged from 0 to 1.0 during the test.

LOW-PASS SPATIAL FlLTERiNG APPARATUSFIG. 2

In step (b) of each cycle of the feedback image enhancement processofthe present invention, a spatial filtering operation is performedwherein the images of the various positives are filtered to attenuatethe high spatial-frequency components thereof. The spatial filtering maybe accomplished in several ways. For example, spatial filtering may beaccomplished in a somewhat crude manner by conventional photographicoptical apparatus by simply defocusing the optics of the apparatus. Theeffect of the defocusing is to blur each line of the image on thepositive into a uniform band in the defocused image. The cross sectionofa blurred line produced by such defocusing is a square. Therefore, thetransfer function of the simple defocusing operation is the Fouriertransform ofa square pulse which is (sin AW)/AW, where W is a frequencyvariable and A is a constant. Since this transfer function providesattenuation of high-frequency data, the defocusing operation representsa low-pass spatial filtering operation. However, the above-mentionedtransfer function has an infinite series of high-frequency peaks whichgradually decay with increasing spatial frequency. These peaks addundesirable high spatial-frequency detail to the defocused image whichcause artifacts to appear in the resultant enhanced image.

To achieve more effective spatial filtering, a line on the originalnegative image should produce a blurred image that peaks at the centerand gradually and monotonically from the center (approximately like andexponential function). This characteristic can be achieved by designinga projection lens so as to have appropriate abberations whereby theblurred image peaks at the center and decays gradually and monotonicallyfrom the center. Another approach is to defocus the optics of aprojection system, and to vary the aperture of a lens included in theprojection system during the exposure by means ofan iris. As theaperture is increased the width of the blurred line increases, and sothe average blurred line can be appropriately shaped.

A more suitable and convenient apparatus for providing effectivelow-pass spatial frequency filtering is shown at 20 in FIG. 2. As showntherein, the low-pass spatial filter mask 22 spaced from the diffuseradiator 21, and a lens 23 spaced from the spatial frequency filteringapparatus 20 comprises a diffuse radiator 21, such as a set offluorescent lamps covered by a frosted glass, a spatial filter mask 22spaced from the diffuse radiator 21, and a lens 23 spaced from thespatial filter mask 22 a distance equal to the focal length of the lens23. The spatial filter mask 22 is a transparency with acircularly-symmetric transmissivity pattern such that the transmissivityis maximum at the center of the mask and decreases gradually from thecenter of the mask to the edge of the mask.

In operation, light rays are directed by the diffuse radiator 21 throughthe spatial filter mask 22. Because of the abovementioned variabletransmissivity pattern of the spatial filter mask 22, a central lightray R illuminating a particular point on the lens 23 has a greaterintensity than a side ray R or a side ray R passing through the mask 22at the outer edges thereof where the transmissivity is less than at thecenter. The lens 23 refracts the incident light so that the central rayR is parallel to the axis of the lens 23 and the side rays R and R,remain slightly divergent. By the above action, a positive transparencyis illuminated by somewhat diffused light in which the perpendicularrays have the maximum intensity and the intensity decreases with anglefrom the perpendicular in accordance with the transmissivity function ofthe spatial filter mask 22. Unexposed film is placed an appropriatedistance below the positive transparency to obtain the desired width ofthe spatial blur. The shape of the blur on the blurred" negative foreach point in the enhanced positive depends upon the transmissivitypattern of the spatial filter mask 22. When spatial filtering isaccomplished with the arrangement 20 of FIG. 2, the blurred image ofaline has a desirable cross section that peaks in the center and decaysgradually at both sides of the center.

Modifications In the practical embodiment of the invention describedhereinbefore, the images in all the steps of the process are recordedphotographically. However, in other embodiments of the invention,different media may be used for storing the imagery data. Nevertheless,these variations of implementation do not alter the principle of theinvention.

For example, the function of the mask be implemented by means ofacathode ray tube, rather than by photographic film. as in the practicalembodiment described hereinbefore. The mask image is then represented bythe two-dimensional pattern of brightness over the surface of thecathode-ray tube rather than by the two-dimensional pattern oftransmissivity values over the transparency that forms the mask in thepractical embodiment described hereinbefore.

As another alternative embodiment, the spatial filtering can beperformed electronically rather than optically. More particularly, thecompressed positive transparency is scanned to form an electronicsignal, and the signal is then processed electronically to form aspatially filtered imagery signal.

What is claimed is:

I. A reiterative feedback image enhancement process which comprises aninitial cycle followed by at least one subsequent cycle and a finaloperation, wherein:

the initial cycle comprises the steps of:

b. deriving from the original image a compressed image the values ofwhich are a fixed function of the corresponding values ofthe originalimage;

c. forming a filtered image by low-pass spatially filtering thecompressed image; and

d. deriving from the filtered image a mask image the values of which area fixed function of the corresponding values of the filtered image;

each subsequent cycle comprises the steps of:

e. deriving from the original image and the mask image derived in thepreceding cycle a compressed image the values of which are a fixedfunction of the product of the corresponding values of the originalimage and the mask image derived in the preceding cycle;

f. forming a filtered image by low-pass spatially filtering thecompressed images derived in the present cycle and all previous cycles,the values of the filtered image being a weighted average of thecorresponding values of the low-pass spatially filtered compressedimages derived in the present and all previous cycles; and

g. deriving from the filtered image formed in the preceding step a maskimage the values of which are a fixed function of the correspondingvalues of the filtered image formed in the preceding step; and

the final operation comprises the step of:

deriving from the original image and the mask image derived in thepreceding cycle an enhanced image the values of which are a fixedfunction of the product of the corresponding values of the originalimage and the mask image derived in the preceding cycle.

2. A reiterative feedback image enhancement process in accordance withclaim 1 wherein the original image and the filtered images are negativeimages and the compressed images, mask images, and the enchanced imageare positive images.

3. A feedback image enhancement process including the steps of:

a producing an original image relating to a scene;

b. deriving from the original image a first compressed image the valuesof which are a fixed function of the corresponding values of theoriginal image;

c. forming a first filtered image by low-pass spatially filtering thefirst compressed image;

d. deriving from the first filtered image a first mask image the valuesof which are a fixed function of the corresponding values of the firstfiltered image;

e. deriving from the original image and the first mask image a secondcompressed image the values of which are a fixed function of the productof the corresponding values of the original image and the first maskimage;

. forming a second filtered image by low-pass spatially filtering thefirst and second compressed images, the values of the second filteredimage being a weighted average of the corresponding values of thelow-pass spatially filtered first and second compressed images;

g. deriving from the second filtered image a second mask image thevalues of which are a fixed function of the corresponding values of thesecond filtered image; and

. deriving from the original image and the second mask image an enhancedimage the values of which are a fixed function of the product of thecorresponding values of the original image and the second mask image.

4. A feedback image enhancement process in accordance with claim 3wherein the original image and the filtered images are negative imagesand the compressed images, mask images, and the enhanced image arepositive images.

5. A feedback image enhancement process including the steps of:

a. producing an original image relating to a scene;

b. deriving from the original image a first compressed image the valuesof which are a fixed function of the corresponding values of theoriginal image;

c. forming a first filtered image by low-pass spatially filtering thefirst compressed image;

cl. deriving from the first filtered image a first mask image the valuesof which are a fixed function of the corresponding values of the firstfiltered image;

e. deriving from the original image and the first mask image a secondcompressed image the values of which are a fixed function of the productof the corresponding values of the original image and the first maskimage;

. forming a second filtered image by low-pass spatially filtering imagebeing the first and second compressed images, the values of the secondfiltered image being a weighted average of the corresponding values ofthe lowpass spatially filtered first and second compressed images;

g. deriving from the second filtered image a second mask image thevalues of which are a fixed function of the corresponding values of thesecond filtered image;

h. deriving from the original image and the second mask image a thirdcompressed image the values of which are a fixed function of theproductof the corresponding values of the original image and the second inaskimage;

i. forming a third filtered image by low-pass spatially filtering thefirst, second, and third compressed images, the values of the thirdfiltered image being a weighted average of the corresponding values ofthe low-pass spatially filtered first, second, and third compressedimages;

j. deriving from the third filtered image a third mask image the valuesof which are a fixed function of the corresponding values of the thirdfiltered image; and

k. deriving from the original image and the third mask image an enhancedimage the values of which are a fixed function of the product of thecorresponding values of the original image and the third mask image.

6. A feedback image enhancement process in accordance with claim 5wherein the original image and the filtered images are negative imagesand the compressed images, mask images, and the enhanced image arepositive images.

7. A reiterative photographic feedback image enhancement process whichcomprises an initial cycle followed by at least one subsequent cycle anda final operation, wherein:

the initial cycle comprises the steps of:

a. exposing a photographic film with illumination falling on a scene anddeveloping the film to produce a scene negative;

b. illuminating the scene negative and exposing a photographic film withthe illumination passing through the negative, and developing the filmto produce a positive;

. illuminating the positive and low-pass spatially filtering the imagethereby formed, exposing a photographic film with the low-pass spatiallyfiltered image, and developing the film to produce a blurred negative;and

d. illuminating the blurred negative and exposing a photographic filmwith the illumination passing through the blurred negative, anddeveloping the film to produce a mask;

each subsequent cycle comprises the steps of:

e. aligning the scene negative with the mask produced in the previouscycle, illuminating the combination and exposing a photographic filmwith the illumination passing through the combination, and developingthe film to produce a positive;

. illuminating the positives produced in the present and all previouscycles and low-pass spatially filtering the images formed thereby,exposing a photographic film with the low-pass spatially filteredimages, the positives being arranged such that the images on thephotographic film are in registration with each other, and developingthe film to produce a blurred negative; and

g. illuminating the blurred negative produced in the preceding step andexposing a photographic film with the illumination passing through theblurred negative, and developing the film to produce a mask; and

the final operation is the step of:

aligning the scene negative with the mask produced in the precedingcycle, illuminating the combination and exposing a photographic filmwith the illumination passing through the combination, and developingthe film to produce an enhanced positive.

8. A reiterative photographic feedback image enhancement process inaccordance with claim 7 wherein:

the gamma of the photographic film used in producing the positive ineach of steps (b) and (e) is equal to the reciprocal of the gamma of thephotographic film used in producing the scene negative in step (a);

the illumination exposure in each of steps (b) and (e) is adjusted suchthat the average value of the relative transmissivity of the positiveproduced in each of steps (b) and (e) is approximately equal to 0.5;

the photographic film in each of steps (c) and (f) is previously foggedby illumination; and

the characteristics of the photographic films used in steps (c) and (d)and in steps (f) and (g) are selected, and the exposure and previousfogging illumination levels adjusted, such that the density at any pointon the mask produced in each of steps (d) and (g) is approximately alinear function of the exposure applied to the corresponding point onthe photographic film in each of steps (c) and (f), respectively.

9. A reiterative photographic feedback image enhancement process inaccordance with claim 8 wherein, in step (f) of each subsequent cycle:

the illumination exposure applied to the positive produced in eachsubsequent cycle is equal to that applied in the previous cycle to thepositive produced in that cycle, and the illumination exposure appliedin each subsequent cycle to each positive produced in an earlier cycleis less than the illumination exposure applied to that positive in theearlier cycle.

10. A photographic feedback image enhancement process comprising thesteps of:

a. exposing a photographic film with illumination falling on a scene anddeveloping the film to produce a scene negative;

b. illuminating the scene negative and exposing a photographic film withthe illumination passing through the negative, and developing the filmto produce a first posime;

e. illuminating the first positive and low-pass spatially filtering theimage thereby formed, exposing a photographic film with the low-passspatially filtered image, and developing the film to produce a firstblurred negative;

d. illuminating the first blurred negative and exposing a photographicfilm with the illumination passing through the first blurred negative,and developing the film to produce a first mask;

. aligning the scene negative with the first mask, illuminating thecombination and exposing a photographic film with the illuminationpassing through the combination, and developing the film to produce asecond positive;

. illuminating the first and second positives and low-pass spatiallyfiltering the images formed thereby, exposing a photographic film withthe low-pass spatially filtered images, the first and second positivesbeing arranged such that the images on the photographic film are inregistration with each other, and developing the film to produce asecond blurred negative;

g. illuminating the second blurred negative and exposing a photographicfilm with the illumination passing through the second blurred negative,and developing the film to produce a second mask; and

h. aligning the scene negative with the second mask, illuminating thecombination and exposing a photographic film with the illuminationpassing through the combination, and developing the film to produce anenhanced positive.

11. A photographic feedback image enhancement process comprising thesteps of:

a. exposing a photographic film with illumination falling on a scene anddeveloping the film to produce a scene negative;

b. illuminating the scene negative and exposing a photographic film withthe illumination passing through the negative, and developing the filmto produce a first positive;

c. illuminating the first positive and low-pass spatially filtering theimage thereby formed, exposing a photographic film with the low-passspatially filtered image, and developing the film to produce a firstblurred negative;

d. illuminating the first blurred negative and exposing a photographicfilm with the illumination passing through the first blurred negative,and developing the film to produce a first mask;

aligning the scene negative with the first mask, illuminating thecombination and exposing a photographic film with the illuminationpassing through the combination, and developing the film to produce asecond positive;

f. illuminating the first and second positives and low-pass spatiallyfiltering the images formed thereby, exposing a photographic film withthe low-pass spatially filtered images, the first and second positivesbeing arranged such that the images on the photographic film are inregistration with each other, and developing the film to produce asecond blurred negative;

. illuminating the second blurred negative and exposing a photographicfilm with the illumination passing through the second blurred negative,and developing the film to produce a second mask; and

h. aligning the scene negative with the second mask, illuminatingcombination and exposing a photographic film with the illuminationpassing through the combination, and developing the film to produce athird positive;

. illuminating the first, second, and third positives and lowpassspatially filtering the images formed thereby, exposing a photographicfilm with the low-pass spatially filtered images, the first, second, andthird positives being arranged such that the images on the photographicfilm are in registration with each other, and developing the film toproduce a third blurred negative;

. illuminating the third blurred negative and exposing a photographicfilm with the illumination passing through the third blurred negative,and developing the film to produce a third mask; and

. aligning the scene negative with the third mask, illuminating thecombination and exposing a photographic film with the illuminationpassing through the combination, and developing the film to produce anenhanced positive.

12. A photographic feedback image enhancement process in accordance withclaim 11 wherein:

the gamma of the photographic film used in producing the positive ineach of steps (b), (e), and (h) is equal to the reciprocal of the gammaof the photographic film used in producing the scene negative in step(a);

the illumination exposure in each of steps (b), (e), and (h) is adjustedsuch that the average value of the relative transmissivity of theexposed positive produced in each of steps (b), (e), and (h) isapproximately equal to 0.5;

the photographic film in each of steps (0), (f), and (i) is previouslyfogged by illumination; and

the characteristics of the photographic films used in steps (c) and (d),steps (f) and (g), and steps (i) and (j) are selected, and the exposureand previous fogging illumination levels adjusted, such that the densityat any point on the mask produced in each of steps (d), (g), and (j) isapproximately a linear function of the exposure applied to thecorresponding point on the photographic film in each of steps (c), (f),and (i), respectively.

13. A photographic feedback image enhancement process in accordance withclaim 12 wherein:

the illumination exposure applied to the second positive in step (f) isequal to that applied to the first positive in step (c), andillumination exposure applied to the first positive in step (f) is lessthan that applied to the first positive in step (c); and

the illumination exposure applied to the third positive produced in step(h) is equal to that applied to the second positive in step (f), theillumination exposure applied to the second positive in step (b) is lessthan that applied to the second positive in step (f), and theillumination exposure applied to the first positive in step (h) is lessthan that applied to the first positive in step (f).

14. A photographic feedback image enhancement process comprising thesteps of:

a. producing a scene positive relating to a scene;

b. illuminating the scene positive and exposing a photographic film withthe illumination passing through the negative, and developing the filmto produce a first negative;

c. illuminating the first negative and low-pass spatially filtering theimage thereby formed, exposing a photographic film with the low-passspatially filtered image, and developing the film to produce a firstblurred positive;

d. illuminating the first blurred positive and exposing a photographicfilm with the illumination passing through the first blurred positive,and developing the film to produce a first mask;

e. aligning the scene positive with the first mask, illuminating thecombination and exposing a photographic film with the illuminationpassing through the combination and developing the film to produce asecond negative;

. illuminating the first and second negatives, and low-pass spatiallyfiltering the images formed thereby, exposing a photographic film withthe lowpass spatially filtered images, the first and second negativesbeing arranged such that the images on the photographic film are inregistration with each other, and developing the film to produce asecond blurred positive;

g. illuminating the second blurred positive and exposing a photographicfilm with the illumination passing through the second blurred positive,and developing the film to produce a second mask; and

h. aligning the scene positive with the second mask, illuminating thecombination and exposing a photographic film with the illuminationpassing through the combination, and developing the film to produce anenhanced negative.

15. A photographic feedback image enhancement process comprising thesteps of:

a. producing a scene positive relating to a scene;

b. illuminating the scene positive and exposing a photographic film withthe illumination passing through the negative, and developing the filmto produce a first negaei c. illuminating the first negative andlow-pass spatially filtering the image thereby formed, exposing aphotographic film with the low-pass spatially filtered image, anddeveloping the film to produce a first blurred positive;

d. illuminating the first blurred positive and exposing a photographicfilm with the illumination passing through the first blurred positive,and developing the film to produce a first mask;

e. aligning the scene positive with the first mask, illuminating thecombination and exposing a photographic film with the illuminationpassing through the combination, and developing the film to produce asecond negative;

. illuminating the first and second negatives and low-pass spatiallyfiltering the images formed thereby, exposing a photographic film withthe low-pass spatially filtered images, the first and second negativesbeing arranged such that the images on the photographic film are inregistration with each other, and developing the film to produce asecond blurred positive;

g. illuminating the second blurred positive and exposing a photographicfilm with the illumination passing through the second blurred positive,and developing the film to produce a second mask; and

h. aligning the scene positive with the second mask, illuminating thecombination and exposing a photographic film with the illuminationpassing through the combination, and developing the film to produce athird negative;

i. illuminating the first, second, and third negatives and low passspatially filtering the images formed thereby, exposing a photographicfilm with low-pass spatially filtered images, the first, second, andthird negatives being arranged such that the images on the photographicfilm are in registration with each other, and developing the film toproduce a third blurred Eositive; j. illuminating the third lurredpositive and exposing a photographic film with the illumination passingthrough the third blurred positive, and developing the film to produce athird mask; and

k. aligning the scene positive with the third mask, illuminating thecombination and exposing a photographic film with the illuminationpassing through the combination, and developing the film to produce anenhanced negative.

16. A reiterative feedback image enhancement process which comprises aninitial cycle followed by at least one subsequent cycle and a finaloperation, wherein:

the initial cycle comprises the steps of:

a. producing a scene positive relating to a scene;

b. illuminating the scene positive and exposing a photographic film withthe illumination passing through the positive, and developing the filmto produce a negative;

c. illuminating the negative and low-pass spatially the image therebyformed, exposing a photographic film with the low-pass spatiallyfiltered image, and developing the film to produce a blurred positive;and

d. illuminating the blurred positive and exposing a photoblurredpositive, and developing the film to produce a mask; each subsequentcycle comprises the steps of:

e. aligning the scene positive with the mask produced in exposing aphotographic film with the illumination passing through the combination,and developing the film to produce a negative;

. illuminating the negatives produced in the present and all previouscycles and lowpass spatially filtering the images formed thereby,exposing a photographic film with the low-pass spatially filteredimages, the negatives being arranged such that the images on thephotographic film are in registration with each other, and developingthe film to produce a blurred positive; and g. illuminating the blurredpositive produced in the preceding step and exposing a photographic filmwith the illumination passing through the blurred positive, anddeveloping the film to produce a mask; and

the final operation comprises the step of:

aligning the scene positive with the mask produced in the precedingcycle, illuminating the combination and exposing a photographic filmwith the illumination passing through the combination, and developingthe film to produce an enhanced negative.

1* i l l araphicfilmwiththe illumination passing through.the.....

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent3.615.433 Dated October 26, 1971 Inv n fl George A. Biernson, RaymondEuling, and Paul W. Jones It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line 32, before "artificial" delete "a" lines 40 and 41,delete "and to attenuate weakly the light from the dark areas of thenegative" line 60, after "simplicity" delete the superfluous commaColumn 3, line 58, before "preceding", insert-the--; line 70,"comprised" should be-comprises Column 4, line 25, after "image"insertA-; line 27, "z should be-Z line 34, "z should be-Z line 42?change "detaif" to-a detailea line 44, after "invention" insert asemicolon Column 5, line 10, the comma after "1.5" should be asemicolon; line 14, "t should be-T line 21, "t should be-T line 22,"coordinat should be p coordinate s line 33, after "2", insert aparenthesis; line 33, "value" should be-values; line 47, in equation (4)"tan h" should be-tanh;' line 48, equation (5) should read Tp= (1/2){l+tanh[2KTp 1.15 log (B/BO) 1};

line 48, after (5) delete "5" line 51, "Log" should be -logline 52,after (6) delete "6" line 67, before "8" insert a parenthesis,- line 69,equation (9) should read 53 3 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3.615.433 Dated October 26. 1971 Inventor(s)George A. Biernson, Raymond Euling and Paul W. Jones Page 2 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

" Column 6, line 5, equation (10) should read -2 D Tp (1/2) (1 tanh[ KTp1 15/ N 0 line 10, "Where" should be-where; lines 12 and 13, "tan h"should be-tanh--; line 16, after 14" insert -where--; line 33, equation(15) should read line 35, "X. should be-x.-; line 35,- "X should be ---Xline 37, "x should bex lin 39, shoigld bex. 1191a 53, "x 81108161 be-xlir le 69, "OF should heofo 0 Column 7, line 2, "T should be'l" line 27,after "cycle", insert a p renthesis; lin 28, after "process", insert a'comma Column 8, line 43, after "FIG. 2", insert a parenthesis; line 47,before "multiple" "is" should be-if Column 10, line 1, "tan h" shouldbe-tanh; line 9, "tan h" should be-tanh--; line 12, equation (23) shouldread Tp= (1/2) l-tanh[ (1/2) log C Ep N -1 Tp [1 C Ep mg? UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Pa nt No- 3,615,433 DatedOctober 26, 1971 Inventor(s) George A. Biernson, Raymond Euling and PaulW. Jones Pa e 3 It is certified that error appears in theabove%dentified patent and that said Letters Patent are hereby correctedas shown below:

line 16, "t should be-T line 19, equation (25) should read p T=[l c E 1line 72, (dD j E should be-- (6D /8E Column 11, line 1, "T should be--Tline 9, "c should be--C line l6, (6D /6E should be ?6D /6E line 24,after insertis to illuminate a clear positive with t he exposure E2 line35, in equation (37) after 2K+l" inser a parenthesis; line 68, before"gradually" insertdecays--; line 69, before "exponential" change "and"to-an- Column 12;, lines 6 and 7, delete "the low-pass spatial filtermask 22 spaced from the diffuse radiator 21, and a lens 23 spaced from"line 12, after 22" insertby- In claim 1, column 12, after line 66 andbefore step "b." insert the step-a. producing an original image relatingto a scene;

In claim 4, column 13, line 74, delete "image being" In claim 11, column16, line 21, before "combination" insertthe-- ggggg UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 3,615,433 Dated October 26,1971 Inventor(s) George A. Biernson, Raymond Euling and Paul W. JonesPACE It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

In claim 13, column 16, line 63, before "illumination" insert--the Inclaim 14, column 17, line 15, after "combination",

insert a comma In claim 15, column 18, line 9, before "low-pass",

insertthe-- Signed and sealed this 25th day of July 1972.

(SEAL) Attest:

2. A reiterative feedback image enhancement process in accordance withclaim 1 wherein the original image and the filtered images are negativeimages aNd the compressed images, mask images, and the enchanced imageare positive images.
 3. A feedback image enhancement process includingthe steps of: a producing an original image relating to a scene; b.deriving from the original image a first compressed image the values ofwhich are a fixed function of the corresponding values of the originalimage; c. forming a first filtered image by low-pass spatially filteringthe first compressed image; d. deriving from the first filtered image afirst mask image the values of which are a fixed function of thecorresponding values of the first filtered image; e. deriving from theoriginal image and the first mask image a second compressed image thevalues of which are a fixed function of the product of the correspondingvalues of the original image and the first mask image; f. forming asecond filtered image by low-pass spatially filtering the first andsecond compressed images, the values of the second filtered image beinga weighted average of the corresponding values of the low-pass spatiallyfiltered first and second compressed images; g. deriving from the secondfiltered image a second mask image the values of which are a fixedfunction of the corresponding values of the second filtered image; andh. deriving from the original image and the second mask image anenhanced image the values of which are a fixed function of the productof the corresponding values of the original image and the second maskimage.
 4. A feedback image enhancement process in accordance with claim3 wherein the original image and the filtered images are negative imagesand the compressed images, mask images, and the enhanced image arepositive images.
 5. A feedback image enhancement process including thesteps of: a. producing an original image relating to a scene; b.deriving from the original image a first compressed image the values ofwhich are a fixed function of the corresponding values of the originalimage; c. forming a first filtered image by low-pass spatially filteringthe first compressed image; d. deriving from the first filtered image afirst mask image the values of which are a fixed function of thecorresponding values of the first filtered image; e. deriving from theoriginal image and the first mask image a second compressed image thevalues of which are a fixed function of the product of the correspondingvalues of the original image and the first mask image; f. forming asecond filtered image by low-pass spatially filtering image being thefirst and second compressed images, the values of the second filteredimage being a weighted average of the corresponding values of thelow-pass spatially filtered first and second compressed images; g.deriving from the second filtered image a second mask image the valuesof which are a fixed function of the corresponding values of the secondfiltered image; h. deriving from the original image and the second maskimage a third compressed image the values of which are a fixed functionof the product of the corresponding values of the original image and thesecond mask image; i. forming a third filtered image by low-passspatially filtering the first, second, and third compressed images, thevalues of the third filtered image being a weighted average of thecorresponding values of the low-pass spatially filtered first, second,and third compressed images; j. deriving from the third filtered image athird mask image the values of which are a fixed function of thecorresponding values of the third filtered image; and k. deriving fromthe original image and the third mask image an enhanced image the valuesof which are a fixed function of the product of the corresponding valuesof the original image and the third mask image.
 6. A feedback imageenhancement process in accordance with claim 5 wherein the originalimage and the filtered images are negative images and the compressedimages, maSk images, and the enhanced image are positive images.
 7. Areiterative photographic feedback image enhancement process whichcomprises an initial cycle followed by at least one subsequent cycle anda final operation, wherein: the initial cycle comprises the steps of: a.exposing a photographic film with illumination falling on a scene anddeveloping the film to produce a scene negative; b. illuminating thescene negative and exposing a photographic film with the illuminationpassing through the negative, and developing the film to produce apositive; c. illuminating the positive and low-pass spatially filteringthe image thereby formed, exposing a photographic film with the low-passspatially filtered image, and developing the film to produce a blurrednegative; and d. illuminating the blurred negative and exposing aphotographic film with the illumination passing through the blurrednegative, and developing the film to produce a mask; each subsequentcycle comprises the steps of: e. aligning the scene negative with themask produced in the previous cycle, illuminating the combination andexposing a photographic film with the illumination passing through thecombination, and developing the film to produce a positive; f.illuminating the positives produced in the present and all previouscycles and low-pass spatially filtering the images formed thereby,exposing a photographic film with the low-pass spatially filteredimages, the positives being arranged such that the images on thephotographic film are in registration with each other, and developingthe film to produce a blurred negative; and g. illuminating the blurrednegative produced in the preceding step and exposing a photographic filmwith the illumination passing through the blurred negative, anddeveloping the film to produce a mask; and the final operation is thestep of: aligning the scene negative with the mask produced in thepreceding cycle, illuminating the combination and exposing aphotographic film with the illumination passing through the combination,and developing the film to produce an enhanced positive.
 8. Areiterative photographic feedback image enhancement process inaccordance with claim 7 wherein: the gamma of the photographic film usedin producing the positive in each of steps (b) and (e) is equal to thereciprocal of the gamma of the photographic film used in producing thescene negative in step (a); the illumination exposure in each of steps(b) and (e) is adjusted such that the average value of the relativetransmissivity of the positive produced in each of steps (b) and (e) isapproximately equal to 0.5; the photographic film in each of steps (c)and (f) is previously fogged by illumination; and the characteristics ofthe photographic films used in steps (c) and (d) and in steps (f) and(g) are selected, and the exposure and previous fogging illuminationlevels adjusted, such that the density at any point on the mask producedin each of steps (d) and (g) is approximately a linear function of theexposure applied to the corresponding point on the photographic film ineach of steps (c) and (f), respectively.
 9. A reiterative photographicfeedback image enhancement process in accordance with claim 8 wherein,in step (f) of each subsequent cycle: the illumination exposure appliedto the positive produced in each subsequent cycle is equal to thatapplied in the previous cycle to the positive produced in that cycle,and the illumination exposure applied in each subsequent cycle to eachpositive produced in an earlier cycle is less than the illuminationexposure applied to that positive in the earlier cycle.
 10. Aphotographic feedback image enhancement process comprising the steps of:a. exposing a photographic film with illumination falling on a scene anddeveloping the film to produce a scene negative; b. illuminating thescene negative and exposing a photographic film with The illuminationpassing through the negative, and developing the film to produce a firstpositive; c. illuminating the first positive and low-pass spatiallyfiltering the image thereby formed, exposing a photographic film withthe low-pass spatially filtered image, and developing the film toproduce a first blurred negative; d. illuminating the first blurrednegative and exposing a photographic film with the illumination passingthrough the first blurred negative, and developing the film to produce afirst mask; e. aligning the scene negative with the first mask,illuminating the combination and exposing a photographic film with theillumination passing through the combination, and developing the film toproduce a second positive; f. illuminating the first and secondpositives and low-pass spatially filtering the images formed thereby,exposing a photographic film with the low-pass spatially filteredimages, the first and second positives being arranged such that theimages on the photographic film are in registration with each other, anddeveloping the film to produce a second blurred negative; g.illuminating the second blurred negative and exposing a photographicfilm with the illumination passing through the second blurred negative,and developing the film to produce a second mask; and h. aligning thescene negative with the second mask, illuminating the combination andexposing a photographic film with the illumination passing through thecombination, and developing the film to produce an enhanced positive.11. A photographic feedback image enhancement process comprising thesteps of: a. exposing a photographic film with illumination falling on ascene and developing the film to produce a scene negative; b.illuminating the scene negative and exposing a photographic film withthe illumination passing through the negative, and developing the filmto produce a first positive; c. illuminating the first positive andlow-pass spatially filtering the image thereby formed, exposing aphotographic film with the low-pass spatially filtered image, anddeveloping the film to produce a first blurred negative; d. illuminatingthe first blurred negative and exposing a photographic film with theillumination passing through the first blurred negative, and developingthe film to produce a first mask; e. aligning the scene negative withthe first mask, illuminating the combination and exposing a photographicfilm with the illumination passing through the combination, anddeveloping the film to produce a second positive; f. illuminating thefirst and second positives and low-pass spatially filtering the imagesformed thereby, exposing a photographic film with the low-pass spatiallyfiltered images, the first and second positives being arranged such thatthe images on the photographic film are in registration with each other,and developing the film to produce a second blurred negative; g.illuminating the second blurred negative and exposing a photographicfilm with the illumination passing through the second blurred negative,and developing the film to produce a second mask; and h. aligning thescene negative with the second mask, illuminating combination andexposing a photographic film with the illumination passing through thecombination, and developing the film to produce a third positive; i.illuminating the first, second, and third positives and low-passspatially filtering the images formed thereby, exposing a photographicfilm with the low-pass spatially filtered images, the first, second, andthird positives being arranged such that the images on the photographicfilm are in registration with each other, and developing the film toproduce a third blurred negative; j. illuminating the third blurrednegative and exposing a photographic film with the illumination passingthrough the third blurred negative, and developing the film to produce athird mask; and k. aligning the scene negatiVe with the third mask,illuminating the combination and exposing a photographic film with theillumination passing through the combination, and developing the film toproduce an enhanced positive.
 12. A photographic feedback imageenhancement process in accordance with claim 11 wherein: the gamma ofthe photographic film used in producing the positive in each of steps(b), (e), and (h) is equal to the reciprocal of the gamma of thephotographic film used in producing the scene negative in step (a); theillumination exposure in each of steps (b), (e), and (h) is adjustedsuch that the average value of the relative transmissivity of theexposed positive produced in each of steps (b), (e), and (h) isapproximately equal to 0.5; the photographic film in each of steps (c),(f), and (i) is previously fogged by illumination; and thecharacteristics of the photographic films used in steps (c) and (d),steps (f) and (g), and steps (i) and (j) are selected, and the exposureand previous fogging illumination levels adjusted, such that the densityat any point on the mask produced in each of steps (d), (g), and (j) isapproximately a linear function of the exposure applied to thecorresponding point on the photographic film in each of steps (c), (f),and (i), respectively.
 13. A photographic feedback image enhancementprocess in accordance with claim 12 wherein: the illumination exposureapplied to the second positive in step (f) is equal to that applied tothe first positive in step (c), and illumination exposure applied to thefirst positive in step (f) is less than that applied to the firstpositive in step (c); and the illumination exposure applied to the thirdpositive produced in step (h) is equal to that applied to the secondpositive in step (f), the illumination exposure applied to the secondpositive in step (h) is less than that applied to the second positive instep (f), and the illumination exposure applied to the first positive instep (h) is less than that applied to the first positive in step (f).14. A photographic feedback image enhancement process comprising thesteps of: a. producing a scene positive relating to a scene; b.illuminating the scene positive and exposing a photographic film withthe illumination passing through the negative, and developing the filmto produce a first negative; c. illuminating the first negative andlow-pass spatially filtering the image thereby formed, exposing aphotographic film with the low-pass spatially filtered image, anddeveloping the film to produce a first blurred positive; d. illuminatingthe first blurred positive and exposing a photographic film with theillumination passing through the first blurred positive, and developingthe film to produce a first mask; e. aligning the scene positive withthe first mask, illuminating the combination and exposing a photographicfilm with the illumination passing through the combination anddeveloping the film to produce a second negative; f. illuminating thefirst and second negatives, and low-pass spatially filtering the imagesformed thereby, exposing a photographic film with the low-pass spatiallyfiltered images, the first and second negatives being arranged such thatthe images on the photographic film are in registration with each other,and developing the film to produce a second blurred positive; g.illuminating the second blurred positive and exposing a photographicfilm with the illumination passing through the second blurred positive,and developing the film to produce a second mask; and h. aligning thescene positive with the second mask, illuminating the combination andexposing a photographic film with the illumination passing through thecombination, and developing the film to produce an enhanced negative.15. A photographic feedback image enhancement process comprising thesteps of: a. producing a scene positive relating to a scene; b.illuminating the scene pOsitive and exposing a photographic film withthe illumination passing through the negative, and developing the filmto produce a first negative; c. illuminating the first negative andlow-pass spatially filtering the image thereby formed, exposing aphotographic film with the low-pass spatially filtered image, anddeveloping the film to produce a first blurred positive; d. illuminatingthe first blurred positive and exposing a photographic film with theillumination passing through the first blurred positive, and developingthe film to produce a first mask; e. aligning the scene positive withthe first mask, illuminating the combination and exposing a photographicfilm with the illumination passing through the combination, anddeveloping the film to produce a second negative; f. illuminating thefirst and second negatives and low-pass spatially filtering the imagesformed thereby, exposing a photographic film with the low-pass spatiallyfiltered images, the first and second negatives being arranged such thatthe images on the photographic film are in registration with each other,and developing the film to produce a second blurred positive; g.illuminating the second blurred positive and exposing a photographicfilm with the illumination passing through the second blurred positive,and developing the film to produce a second mask; and h. aligning thescene positive with the second mask, illuminating the combination andexposing a photographic film with the illumination passing through thecombination, and developing the film to produce a third negative; i.illuminating the first, second, and third negatives and low-passspatially filtering the images formed thereby, exposing a photographicfilm with low-pass spatially filtered images, the first, second, andthird negatives being arranged such that the images on the photographicfilm are in registration with each other, and developing the film toproduce a third blurred positive; j. illuminating the third blurredpositive and exposing a photographic film with the illumination passingthrough the third blurred positive, and developing the film to produce athird mask; and k. aligning the scene positive with the third mask,illuminating the combination and exposing a photographic film with theillumination passing through the combination, and developing the film toproduce an enhanced negative.
 16. A reiterative feedback imageenhancement process which comprises an initial cycle followed by atleast one subsequent cycle and a final operation, wherein: the initialcycle comprises the steps of: a. producing a scene positive relating toa scene; b. illuminating the scene positive and exposing a photographicfilm with the illumination passing through the positive, and developingthe film to produce a negative; c. illuminating the negative andlow-pass spatially the image thereby formed, exposing a photographicfilm with the low-pass spatially filtered image, and developing the filmto produce a blurred positive; and d. illuminating the blurred positiveand exposing a photographic film with the illumination passing throughthe blurred positive, and developing the film to produce a mask; eachsubsequent cycle comprises the steps of: e. aligning the scene positivewith the mask produced in the previous cycle, illuminating thecombination and exposing a photographic film with the illuminationpassing through the combination, and developing the film to produce anegative; f. illuminating the negatives produced in the present and allprevious cycles and low-pass spatially filtering the images formedthereby, exposing a photographic film with the low-pass spatiallyfiltered images, the negatives being arranged such that the images onthe photographic film are in registration with each other, anddeveloping the film to produce a blurred positive; and g. illuminatingthe blurred positive produced in the preceding step and exposing aphotograPhic film with the illumination passing through the blurredpositive, and developing the film to produce a mask; and the finaloperation comprises the step of: aligning the scene positive with themask produced in the preceding cycle, illuminating the combination andexposing a photographic film with the illumination passing through thecombination, and developing the film to produce an enhanced negative.